Mars may have been a much wetter and more geologically active world than it is today, with water repeatedly coursing through its rocks in different forms, a new study suggests.
The discovery comes from a region along the edge of Mars' Jezero Crater known as the Margin Unit. NASA's Perseverance rover has been exploring the area since 2023, and its observations reveal evidence of at least three distinct episodes of water interacting with Martian rocks — including one involving hot groundwater circulating beneath the ancient surface, according to a statement from the space agency.
The Margin Unit region hugs the inner edge of the crater and was identified through observations from orbit. Scientists initially expected Perseverance to encounter sedimentary rocks deposited along the shoreline of the ancient lake that once filled Jezero.
Instead, the rover found igneous rocks that originally formed from molten material deep underground or through volcanic activity. That unexpected geology preserved an intricate record of water activity.
"Before we arrived at the Margin Unit, the main hypothesis — derived from orbital observations — was that the carbonate seen from orbit formed from interaction with the lake that existed in Jezero Crater," Candice Bedford, lead author of the study and a research scientist at Purdue University in West Lafayette, Indiana, said in the statement. "But now we know that this location became a sort of crossroads for aqueous systems. The Margin Unit findings are important because Jezero Crater sits inside one of the largest exposures of carbonate on Mars, so what we learn here reaches well beyond this crater."
Using Perseverance's SuperCam instrument, researchers analyzed more than 185 bedrock targets across the Margin Unit. They found evidence that water altered the rocks during multiple episodes.
During the first episode, carbon-dioxide-rich groundwater reacted with olivine, a mineral that formed as magma slowly cooled. The reaction produced carbonate minerals within fractures in the rock.
A later episode appears to have involved Jezero's ancient lake. Some of the rocks contain silica, which can form when olivine reacts with water and is particularly abundant in rocks that were once below the lake's waterline.
Then came a third, hotter episode. Mineral veins containing calcium sulfate and fluorite formed in the eastern Margin Unit, suggesting that hot water once circulated through the volcanic rocks.
On Earth, hydrothermal systems can create environments capable of supporting microbial life, making these ancient Martian water systems particularly interesting to scientists investigating the Red Planet's past habitability.
While the researchers are able to reconstruct the sequence of these episodes, they cannot yet determine exactly when each occurred. However, the order of events suggests the Margin Unit was more than simply an ancient lakeshore. It was a place where groundwater, surface water and heated fluids interacted with Martian rocks, potentially preserving a complex record of how Mars' environment evolved over time — one that the carbonate minerals in these rocks could help scientists use to determine when and where Mars may once have provided conditions suitable for life.
"If there is one thing I have learned after 10 years working with Mars rovers, it is that Mars constantly throws surprises at you," Bedford said in the statement. "It is very rare that things are as we expect them to be from orbital data. I hope this work helps reshape how scientists view the history of water in Jezero Crater and across Mars. Ultimately, I hope it helps planetary scientists reconstruct the changing climate and habitability of early Mars."
Their findings were published September 21 in the journal Communications Earth & Environment.
